The purpose of the proposed study is to collect basic data on the intraspecific aggressive behavior (agonistic behavior) of female housemice (Mus musculus). Ebert found that, after rearing in isolation, 25% of a population of wild-trapped female mice attack a C57BL intruder. Artificial selection for this agnostic behavior was begun and is currently at generation S9. The proposed work involves continuing the selection program and doing 10 related experiments. Maternal aggression, non-isolation-induced aggression, male aggression, and intersex aggression will all be investigated as correlated characters. The development of female aggression will be studied. The relationship between reproductive fitness and agonistic behavior for females will be investigated. Maternal effects on aggressive behavior will be investigated using cross-fostering. Two experiments will investigate physiological influences by looking at estrous cycle fluctuations in agonistic behavior, and endogenous endocrine characteristics of the selection lines (plasma testosterone levels and testosterone uptake in the brain).